Note: Descriptions are shown in the official language in which they were submitted.
~37Q1~7
PROC-SS FOR CALCINING COKE
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: BACKGROUND OF THE INVENTION
The present invention rela~es generally to a :
process for calcining~green coke obtained by~a de-
layed coking process and~more~speclf1cally to~a
: proaess for producing~wlth~a~h:igh~thermal~eff~iciency : :~
~ hlgh-grade;coke suitable for~use~ln:the~:prepàration~
; ~ : of:~raphite~electrodes.~
: Preparation:of green coke from heavy olls of
:: petroleum origin:such as residue oils of catalytic
:
cracking and thermal cracking, stral~ht run residue
olls and tar~;;of~thermal cracklng,~coal~tar;p~ltoh~or~
mixtures thereo~by a delayed coklnq proces~s is kncwn : : : ~-- -
in the art. The~green coke prcduced by this process ;~;
stil~l contains a~significant~:~uanti~y:o~ moisture and~
volatile matter. ~Accordingly, there is also known a
process Eor calcinlng~ hc~produced~green coke i~
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order to.remove the moisture and volatile matter from
the green coke and to densify the same, thereby pro-
ducing a carbon material having a high density and a
low coefficient of therma~ expansion which is suitable ~ .
for use as an electrode material fox steel~making,
aluminum smelting or the like or a carbon material for ~ :.
othèr shaped articles.
Calcining of such green coke is carried o~t in
h~ating furnaces such as a rotary kiln~ a rotary hearth,
and a shaft kiln in a single stage, or in two stayes
by further providing a preheating furnace.
W~ have found, as a result o our research on the
unit stages in the calcination of coke, that one or
two stages of heating furnaces are insufficient ~nd at :~
least three stages of heating furnaces are necessary
in order to produce high~grade coke efficlently, and :
have developed a pxocess for calcining coke (Japanese
Patent Laid-Open Publication No. 10301~1979, U.S. Pa-
tent No. 4,169,767. More Particularly,
this calcining process comprises calcining green coke
sbtained by a delayed coking process in heating fur-
naces of at least:three stages connected in series, in
which the control of temp~rature and the adjustment of
a~mosphere in the respective furnaces can be indepen-
dently carried out, which process comprises carrying
aut the following steps in the respective furnaces in
the indicated order:
.
. ~
a) evaporating the water contained in the
green coke, and drying and preheating the coke;
b) distilling off and burning the volatile
matter in the dried coke; and
c) heating and calcining the coke from -the
step b).
As ar as we are aware, the calcined coke obtain-
ed ~y this process is not fully satisfactory as coke
for artificia1 graphite electrodes which needs to be
of particularly hlgh grade. That is, there remains
much room ~or improvement with respect to high density
and low coefficient of thermal eXpansiQn which are
the most important properties required of coke for
artificial graphite electrodes.
On the other hand, our research staf~ has found
that cooling in an intermediate stage in the calcina~
~ -
tion of coke is highly effective~ in reducing the co- ~
: ~ ~
efficient of thermal expansion of the calcined coke
and increasing the density, particularly the true den-
sity thereof, and has developed a process for producing
high-grade coke. This process for calcining coke com-
prises first calcining green coke obtained by a delayed ~ -
coking process at a~temperature lower than an ordinary
calcining temperature, cooling once the calcined coke,
and thereafter calcining the coke again at a tempera- `~
ture in the ordinary calcining temperatuxe range ~as
disclosed in U.S. Patent No. 4,100,265, July 11, 1978).
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Although it is not suficiently cleax why the coef-
~icient of thermal expansion o~ the calcined coke is
reduced by intPrmediate cooling, a possible reason
may be that some fine cracks are foxmed in the coke
during the process wherein the coke, after being
heated to a temperature of 600 to 1000C, is ~ubject-
ed to intermediate cooling and then to reheating,
which cracks are considered to absorb expansion due
to heating, resulting in the reductio~ of the overall
coefficient of thermal expansion of the coke. The true
density o~ the calcined coke is increased presumably
b~cause rapid evaporation of volatile matter and for-
mation of a porous structure which occurs as a result
;thereof axe suppressed by the lntermediate cooling in
the above specified temperature range.
jIt may appear that calcinec coke o~ higher grade
can be ohtained by applylng this concept to the~pxo-
cess disclosed in the aforementioned U.S. Patent
No~ 4,169,767 (hereinafter re~erred to as
"original three~stage process~"), i.e., by once cooling
the preliminarlly calcined coke from the step b) in
the orlginal three-stage process, and then oaLci~ing
the coke in the step c). However, this is not as
easy as might be expected because, ~or once cooling
t~e preliminarily calcined coke from the step b),
reh~ating the coke to a temperature equal to the
tem~erature of the outlet of the furnace for the step
.
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~L37(~ 7
b), and further supplyirlg heat re~ulred ~or the final
calcination, the heating furnace for the step G) will
be too heavily loaded, and the quantity of the sen~-
sible heat of waste gas obtained by the increased
load will be so great that it cannot be consumed in
the en-tire calcination system. Thus~ the application
of the original -three-stage process to the two-stage
calcining process according to the aforementioned
U. S. Patent No. 4,100j265~which comprises inter-
mediate cooling has beerl ~onsidered unpractical.
As a result of our ex:tended:researchr however, :~
it has been found that,:by suppressing to a min~imum ~
the combustion of the::vola~til~e matter evaporated in the
step b~) .in the original three-staye process described
above, and, lnste:ad of using~the waste gas from the
step b) as a gas for:~drylng~and preheating the coke
in the step a)~as:in~the oriyinal three-stage~process,
introducing~this waste gas into the step~:c) where~the .:
gRS is burned~and utlllzed as~a heat source:for~the :
final calcination o;the:coke,~the overall sensi:ble
heat of the~waste gas :is not greatly increased,~even
i~ the heat load in the step c) is increased, and thus
can be utilized inthe system.
It has further been:found that, by suppressing
the ~ombustion of~the~vo.latile matter in the step b),
it becomes easier to control the temperature at the
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outlet through which the calcined coke from the step
b~ is discharged. It should be noted that this con~
trol is the most important problem encountered in the
two-staye calcining process. In order to suppress
the combustion of the volatile matter in the step b)l
it is sufficient to introduce air only in the minimum
quantity required for the combustion of ~uel to gene-
rate heat necessary for the preliminary calcination
in the step b) and to maintain the system under a
non oxidizing atmosphere;.
SUMMP~RY OF THE INVENTION
On the basis of the above considerations, the
present invention aims at improvements in a two-stage
calcining process COmprlSing intermedlate cooling so
that the pro~ess can be~ practiced on a~commercial
sc~le.
More 9peGlically~ the process for calcining coke
according to the present invention is a process for
calcining green coke~obtained by a delayed coking
process in heat~ing furnaces of at least three stages
connected in series, in which the control of tempera~
ture~and~th~adjustment of~atmosphere in the respec-
tive furnaces can be independently carried out, which
process comprises carr~ing out the following steps in
the respective fu~rnaces in the lndicated order~:
a) evaporating the water contained in the
green coke, an~ drying and preheating the coke;
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b) distilling off the volatile matter from
the dried coke; and
c) burning the volatile matter from the step
b), and calcining the coke,
the coke from the step b), after being once cooled,
being introduced into the step c). `-~
r~he present invention will be further described
with respect to an example with reference to the ac-
companying drawing which shows an example wherein ~ : ;
rotary kilns are used as heating furnaces~
BRIEF DESCRIPTION OF THE DR~WING
In the drawing~
FIG. 1 is a flow chart illustrating one example ~ ` :
of the process of the present invention using rotary
kilns as heating furnaces,~ wherein solid lines, one~
dot chain lines, a:two-dots::ohaln line,~ and a~broken ;~
line:respectively indica~te passages for coke, pre- :
heated air, waste gas containing volatile matter, and
burned waste:gas; and:
.
FIG. 2 is a partial side view illustrating an
: arrangement of a raw mateIial:feeder 1 provided in a
kil~ 2~
DETAILED DESCRIPTION ~ :
The numerical values set: forth hereinafter are
only ~ypical ones, and, ln particular, the tempera~
ture and retention time values indica~e standard ranges.:
Of course, these values can be appropriately varied
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depending on the proper-ties of green coke and -the pro-
perties of the calcined coke desired.
Referring to Fig. 1, the green coke obtained by
a delayed coking process is dressed into the desired
particle size distribution, for example, such that
about 25~ is not greater than 3 mesh, about 75g6 is
about 3 mesh, and the maximum particle diameter is not
greater than 70 mm. Then, the coke is introduced into
a drying and preheating kiln 2 through a raw material
feeder 1.
The raw material feeder 1 may be of a type where-
in a hopper chute is directly inserted into the kiln
from the upper end thereof. In order to ensuxe a bet-
ter airtightness, as is shown in Fig. 2, it i5 pre-
ferable that the feeder be of such a type that raw
materlal coke~is lntroduced into an annular raw material
reservoir lc having a diameter greater than that of the
,
kiln body 2b in the neighbourhood of the upper end 2a
of the kiln, through a conveyor la and a hopper chute
lb, and a trough ld communicating with the kiln body
2b is provide?, for example, at four por-tions within
the reservoir lc~ The raw material is charged into the
kiln through the troughs.
The green coke typically has a water content of ?
to 10% ~by weight, as in all percentages hereinafter),
a volatile matter content of 6 to lOgo (according to
JIS M~812~ and an apparent density of 0.80 to 0.95
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g/cm3. The green coke in the kiln 2 is heated to a
temper~ture of 300 to 4Q0C by a hot gas (which is
at a temperature between about 900 to 1,200C)
introduced into the kiln 2 through a duct 5 from a
final calcining kiln 4 as hereinafter described. As
a result, preheating of the coke i5 carried out with
evaporation of the water.
The inclination angle of the kiln 2 is of the
order o~ 1.2 to 3.0 degrees,~ and the inner diameter,
the total le~gth, and the rotational speed of the
kiln are selected~ so as to ensure a retention time
of 10 to 30 minutes. By way of example, an inner
diameter of 2.3 m, a total length o 20 m, and a
rotational speed of b. 5 to l.O rpm~are adopted for a
green coke charge o~ 10 tons/hr.
The hot gas~leaving~the kiln 2 is stlll at a~
temperature~of about~400 to 600C,~which gas is in~
troduced into an air preheater 7 through a duct 6
,' where the gas undergoes a heat exchange with air,
I
and the gas itself is cooled to a temperature of about
200 to 300C and then discharged outside of the system
~through a chimney~8, while the air is preheated to a ?
temperature of 200 to 400C. The preheated air is
introduced into the combustion chamber 10 of a pre-
liminary calcining kiln 3 and the combustion chamber
.
11 of the final calc.ining kiln 4 through a piping
(9a, 9b). Further, an air inlet ~not shown) is
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provided at the base of the chimney 8 50 as to con-
trol the quantity of air introduced thereby to adjust
the pressure in the chimney, for example, to -20 mm
H2O, or, if desired from the standpoint of -the pres-
sure balance between the respective parts in the
system, an induced draft Ean is provided in the
duct midway between the outlet of the air preheater
through which waste gas is disaharged and the chimney.
The coke preheated to a temperature of 300 to
400C in the drying and preheating kiln 2 is lntro-
duced into the preliminary calcining kiln 3 through
a coke feeding device 12 where fuel is burned by a
burner 13 with the preheated air from the piping 9a,
and the volatile matter is dLstilled off the coke by
heat due to the combustlon~of the fuel, the coke be-
lng~heated to a temperature of~about 600~to l,000C.
In this temperature range the volatile matter ¢on-
tained in the coke is dispersed, and the coke rapidly
shrinks. Thusj~ whether or not the temperature of the
preliminary calcining kiln is ma:intained and control-
led in thi6~range critlcally affects the quality of
the product coke. ~ ~
The coke feeding devioe 12 ls o~ almost the same
type as the raw material feeder l. Ordinarily, the
inlet end of the kiln 3 is disposed immediateLy below
the outlet end of the kiln 2, and the preheated coke
:
from the kiln 2 is directly dropped by gravity into
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an annular raw material reservoir 12c (not shown,
corresponding to the reservoir lc of Flg. 2) of the
coke feeding device 12 oE the kiln 3 through a con-
duit. If such an arrangement i5 not appropriate,
the transportation between the kilns may be carried
out by means of a steel belt conveyor or a moving
hopper.
The combustion chamber 10 has a construction in
which the discharge opening for the combustion gas is
connected directly to the outlet of the~kiln. A bur- -
ner which can be~used as the burner 13 is not l~imited
with respect to fuel and type. Particularly, a short
flame premixing type gas burner wherein a fuel gas
and air for combustion are uniformly mixed, and;the
mixture is injected through a nozzle for combustLon
thereof is pre~erable for the reason~that wasteful com-
bustion of the coke~and the voLatiIe matter can be
~avoided.
The quantit:y~of ~he preheated air introduced into
the comhustlon chamber 10 from the piping 9a is con-
trolled within th~ range of from the minimum quantity
required for the combustion of fuel up to 10~ ln ~ ;
excess thereof~so as to maintaln the kiln 3 under a
substantially non-oxidizing atmosphere and minimi2e
the combustion of fuel. Further, in order to prevent
the formation of ring-shaped adhesive materials (coke
ring), regular or irregular shapes and arrangements of ;~
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31~3~7 ~ ~:
lifters on the surface of the insulating refracto- :
ries may be provided within the kiIn, and:the coke
is therehy agitated ancl heated as thoroughly as
possible to prevent the aggregation and adhesion of
coke particles due to the volatile matter, thus sup-
pressing the formation of ring-shaped adhesive
materials.
The inclination~angle of t:he kiln 3 is~about
1.2 to 3.0 degrees, and an approprlate reLention ; :~
. time is between about 3G and 90 minutes. :The flow ~ ;
i direction of combustion gas is not limited~to~a counter
flow relative to that of the coke as shown in Fig. 1
but may be a parallel o~r concurrent flow. Nowev~r,
in order to increase the thermal eLficlency thereby to
dlstill off the volatile matter efficiently:in an in~
termediate zone ;and~;to;:control the coke~calci.ning~tem- :
: perature, a:oount r flow as~:shown in the: figure is
~prefer~bLe. ~
Then,~the~coke heated-to a temperature of about
600 to 1,000C is withdrawn through a withdrawAl
device 14 of the kiln 3 and introduced into an inter-
mediate cooling zone l:S~where~ the~coke is subjected
to:natural coollng or forced cooling, for example, by :~
spraying with water to a temperature of from room
temperature to 200C, pr~e~erably to a temperature no~
exceeding 100C. In order to prevent the oxidation
of the coke in the coollng process, the coollng rate
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is preferably controlled to be not lower than 100C/hr.
The coke thus cooled is introduced into the final
calcining kiln 4 through a coke feeding device 16.
On the other hand, the waste gas containing the
volatile matter from the kiln 3 is intxoduced into
the combustion chamber 11 of the kiln 4 through a
piping 17, where the waste gas is burned by the pre-
heated air from the piping 9b, and the combustion gas
.
is utilized to heat the coke introduced into the kiln
4 to a calcining tempera-ture of 1,200 to 1,500C for
calcination. In order to completely burn in the
combustion chamber 11 the waste gas containing the
volatile matter from the piping 17, the waste gas is
thoroughIy mixed with alr by blowing the gas in such
a manner that the gas stream contacts the stream of
the preheated a~r in-troduced through the pip~ng 9b
perpendicularly thereto in the combustion chamber 11
or that the waste gas creates a turbulence within the
combustion chamber 11.
The combustion chamber 11 is provided with a
burner 18 for burning ordinary fuel which is used at
the start of the operation and also for~heating auxi-
liary fuel required for the control of temperature.
The kiln 4 is inclined at an angle of 1.2 to 3.0
degrees, and the total retention time of the coke is
between 30 and 90 minutes. In this kiln 4, the coke
is maintained at the calcining temperature for about
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10 to about 30 minutes~
The calcined coke is withdrawn as a product through
a withdrawal device 19 positioned before the combus
tion chamber 11. On the other hand, the waste com-
bustion gas from the kiln 4 is introduced into the
dryiny and preheatiny kiln 2 through the duct 5, and
utilized as a heat source~ Ordinarily, the withdrawn
coke is introduced into a cooler of ro-tary kiln type
which is provided with a spray nozzle for cooling water
therein and cooled b,y water directly sprayed thereon.
If desired, the coke may be cooled by a gas.
The flow~rate and temperature distribution res
pectively at various parts per ton o~ yreen coke are
shown in the followiny table by way of example.
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tion Flowing material Temperature Flow
No, (C~ quantity
. _
1 Green coke Room tem- 1 ton
perature
12 Praheated coke 400 0.97 " ~ .
: 14 Preliminarily calcined300 0.86 "
coke
16 In-termediately cooled 80 0.86 ~'
co]ce
lg Flnal calcined co]ce1,350 0.85 "
_ :
9 Preheated air 250 1,~03 Nm3
9a .- ll 247 "
9b .l 956 "
.
17 Waste~gas con-taining 900 337 " :
volatile matter ~
5: Waste combustion gas1,000 1,301 "
6 : ~ " ~20 1,305 " ~:
. ~ _...... . ~ ~ ,'
13 Fuel (calorific value _ 25 Kg
8,800 kcal/kg) ~.
8 ~ _ - _. 6 " :-
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The typical properties oE -the calcined coke thus
obtained and those of the calcin~d coke obtained with-
out intermedlate cooling are shown below.
With in~erme- Without inter-
diate coolin~ mediate coolln~
~pparent density(g/cm3) 1.42 1.42
True density (g/cm3~ 2.169 2.110
Coefficient of thermal
expansion* (roast~d 1.1 1.2
at l,000C)~x l0-~/C)
Coef~icient of thermal
expansion*(graphi~tized 0.7 0.8
at 2j600C)(x 10-6/C~ ~
* The coefflcient of linear thermal expansion was
determined as follows.
The calclned coke was pulverized, and 92% of -the
particles having a particle~size of above 200 mesh and
:
8% of the particles~having a particle size below 200
mesh were mixed. 100 parts of this mixture was mixed
:
with 25 parts of coal~tar binder pltch (of a softening
point of 90.3C, a benzene lnsoluble content of 19.8%,
a quinoline lnsoluble content of 4.4%, a volatile
matter content of 62.7%, and a ~ixed carbon content of
,
~53.2%), and the mixture was heated, kneaded and mold-
shaped into two~mol~ded articles, of which one was
roasted at l,000C, and ths;other was graphitized at
2,600C. Test pieces (rods~5 mm in diameter and about
50 mm in length)~;prepared from these molded artLcles
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were tested at temperatures over a range of 30 to
100C.
I~ the above described example, a rotary kiln
w~s used for each of the three heating furnaces.
However, a part or all of these rotary kilns may
al~o be su~stituted by a rotary hearth, a retort, or
a shaft kiln. It is preferable, however, that a
xotary kiln be used or each of the preliminary cal-
cining kiln and the final aalcining kiln for the rea-
j sons that the combustion of the volatile matter can
j be suppressed, ~hat uniform calcirlation of coke can
be carried outj and that the process operation can
b~ facilitated.
In addition, it is most preferable to use threeheating furnaces from the standpoint of apparatus
economy while the independent controllability of the
respective furnaoes is maintained. If necessary,
however, the respective stages or steps can be, of
course, further divided into stages or steps with a
pluxality of furnac~s.
As is apparent from the foregoing, the process
for calcining coke according to the present in~ention
has the following ad~antages.
~1) By maintaining the independ~nt states
o~ tha respectlve stages achieved by the three-stage ~ ;~
proces~ disclosed in U~S~ Patent NOr . . .
4,169,767 and controlling the respective stages of the
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green coke calcination independently from each other, ~ .
the optimum conditions for producin~ high grade
coke can be realized while wasteful combustion of the
produc~ coke can be suppressed.
(2) By adopting intermediate cooling, it is .
po~sible ~o produce high-grade coke which is most ' .
~uitable for use as a graphite electrode.
~ 3) By utilizing the heat o combustion of
the volatile matter effectively in the system:~ the
overall increase in quantity of fuel used can be
controlled within a reasonable range in spite of the
adoption of Lntermediate cooling. For example, the
quantity of fuel used can be reduced by about 60% in
comparison with ~hat required when the coke is sub-
jected to intermediate cooling between the second and
third steps in the process disclosed in the:afore- . :
mentioned U.S.~Patent No. 4,169,7670
Thus, the most advantageous feature of the pre- .
sent invention resides in that it has succeeded in the
commercialization of a two~stage calcining process
comprising intermediate cooling, which has be~n dif-
.
i ficult to realize because of the limitations from the
~tandpoint of economy, particularly of heat economy,
although coke of high quality can be obtained thereby. :~
~ urther, the above described apparatus for use
for the process of the present invention can also be
used for a process for calcining c~ke comprising ~o ::~
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intermediate cooling. Althouyh the quality of the
product coke is sacrificed in such a case, improved
thermal efficiency and operation conditions are
maintained, and better results can be obtaineAd even
with respect to the quality of the product coke as
eompared with the eonventional process for calein-
ing eoke using one or two urnaces.
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